Now that Sam Nunn has lost his title as every Republican's favorite Democrat, many in Washington seem anxious to shift that title to Lloyd Bentsen - but the wily old Texan is not so sure he wants it.

Bentsen did finally cast one of only three Democratic votes in support of fellow Texan John Tower, but the Democrats' 1988 vice presidential candidate tells me that in economic affairs, where his influence may be the strongest in the Senate, he is determined to chart a highly independent course.In the end, that could in fact add to his power to affect the nation's pocketbook, a power that is already, ironically, considerably greater than it would have been if he had won the job for which he was nominated last year. (As the old story put it, a woman had four sons; one became vice president of the United States, and nothing was ever heard from the other three, either.)

Bentsen, at 68, may be getting too long in the tooth to achieve his own presidential ambitions, and that alone could add to his credentials as a symbol of something long missing in Washington: intelligent bipartisanship in economic policy.

One obvious example is legislation to promote capital investment, which when all the partisan shouting has evaporated remains the most certain key to better future jobs for Americans. Dan Rostenkowski, who as House Ways and Means chairman is the counterpart of Bentsen as Senate Finance chairman, did his best to torpedo a cut in capital-gains taxes by demagogically threatening to raise the top percentage income-tax bracket in retaliation.

That has led to Washington reports that President Bush, whose proposal for a 15-percent rate on capital gains has so far been treated with disdain, would revive the proposal by making Bentsen his point man on Capitol Hill. Bentsen says, in effect, no thanks.

Such an alliance would be historically fascinating. Not only was Bentsen half of the team that Bush-Quayle defeated last year, but Bentsen first won his Senate seat in 1970 by beating . . . George Bush. Against all the odds, they have remained remarkably civil.

And it's unmistakably true that, on economic policy, Bentsen and Michael Dukakis were the odd couple of the decade. The Texan did not demur when I asked him the other day how hard he had to bite his tongue during the 1988 campaign while the head of his ticket "talked as if reducing the capital-gains tax was a class-war issue that would benefit only the rich."

Bentsen acknowledged "in all candor" that he had indeed worked hard over the years to reduce that tax, and he maintained that the results had been "effective" and "helpful."

But he's not about to let the Republicans off the hook, either. He noted acerbically that the Reagan administration itself urged the 1986 increase in the capital-gains rate, asserting that the change would "pick up $21 billion in revenue," and that there was thus something peculiar about the Bush team's now claiming that a reduction in the same tax would "pick up $16 billion." The Republicans "just can't have it both ways," Bentsen declared.

So, while his heart obviously is with Bush's arithmetic rather than Reagan's, Bentsen says he's going to withhold even his own personal decision until he sees more convincing evidence that reducing the capital-gains tax - without raising other rates - will truly shrink, rather than enlarge, the federal deficit.

Bentsen's overriding concern, he insists, is that deficit. And he does not conceal his contempt for what he regards as Bush's unrealistic economic assumptions, notably in simultaneously forecasting robust growth and lower interest rates.

Asked for his own budget-cutting suggestions, Bentsen mentioned "bring(ing) more order to the purchasing power of the Pentagon," reducing the need for agricultural subsidies by opening foreign markets, and getting the Japanese and Western Europeans to absorb more U.S. defense costs.

All good bipartisan ideas, to be sure. Yet when I asked Bentsen whether there was "any existing social program that we don't need at the current level of funding," he replied after a pause that he couldn't think of any. Lloyd Bentsen may indeed be one of the few members of Congress who actually can add. But even he doesn't seem terribly good at subtraction.